A fluorescent material emits light of a certain wavelength in response to absorbing light of a different (and typically shorter) wavelength. Fluorescence thermometry may be utilized to measure the temperature of fluorescent materials, and indirectly measure a second material in contact with the fluorescent material, by taking advantage of the temperature-dependent fluorescent behavior of certain materials. Generally, some characteristic of fluorescence (intensity, decay time, wavelength) is correlated with temperature. As an optics-based technique, fluorescence thermometry lends itself to the use of optical waveguides, particularly elongated waveguides such as optical fibers, light pipes, and the like. Thus, active components utilized in fluorescence thermometry (excitation source, optical sensor, electronics) can be placed remotely from the surface to be measured. Accordingly, the temperature measurement can take place in an environment unsuitable for these active components, such as in the presence of high temperatures, harsh chemicals, or electromagnetic fields.
Several approaches have been developed for implementing fluorescence thermometry. The most successful approach to date entails correlating fluorescence decay time with temperature. This approach overcomes problems with accuracy and stability, but requires sophisticated and costly components, including a modulated light source and suitable electronics to extract fluorescence decay time. By measuring decay, where lower signal levels contribute to the measurement, noise can be a considerable factor. Also, because it is a decay process where many cycles must be averaged to determine a decay time, response time is typically limited to about 0.25 seconds or more.
An alternative approach to optical thermometry involves measuring the ratio of emission between two fluorescent materials, such that the ratio is correlated with temperature. These approaches are optically inefficient. One example is the use of separate fibers or a split fiber, together with separate optical sensors and optical filters. In such an approach each optical sensor receives less than half of the photons generated by the fluorescence process. Another example is the use of a grating to distinguish wavelengths, which is even more optically inefficient due to the spatial dispersion of light.
There is an ongoing need for fluorescence-based thermometric apparatus and methods that are more optically efficient, accurate, reliable, and able to respond quickly. There is also a need for fluorescence-based thermometric apparatus and methods that are operable in adverse environments.